The present invention is directed towards a superconducting glass-ceramic material and method for making the same. The composition of this glass-ceramic material is derived from the Bi-Ca-Sr-Cu-O family of superconductors. In particular, it has been discovered that when the Ca to Sr combination is maintained in certain proportions and, preferably, controlled additions of ZnO are provided, that relatively stable glasses crystallizable to glass-ceramics with good superconducting properties can be produced.
Further, with appropriate heat treatment, a particularly desirable crystal morphology, also observed in Bi-Ca-Sr-Cu-O superconductors, can be developed in these stable glasses. This is a morphology comprising platelets or crystallites with an angularly juxtapositionally disposed orientation that exhibit very good superconductivity properties in combination with low room temperature resistivity.
It is well known to those skilled in the superconductor art that this fast paced technology is in need of a composition and process whereby useful products can be manufactured. The ceramics of the first discovery, by Bednorz and Muller created excitement and happily spurred research, due to a significant raising of the critical temperature of superconductivity in their ceramic discovery.
The yttrium-barium-copper oxide (1:2:3) superconductor of the Chu et al. discovery, while exhibiting superconductivity at higher temperatures, does not exhibit mechanical strength, useful crystal orientation, nor sufficient crystal positional proximity for useful products. The 1:2:3 superconductivity discovery cannot be trivialized, however, since this discovery has inspired others to invent additional ceramic superconductive compositions.
Recently, Maeda et al. in the Japanese Journal of Applied Physics 27, L209, (1988), disclosed the discovery of a new family of ceramic superconductors. This family is known as the Bi-Ca-Sr-Cu-O ceramic superconductors. The particular member of that family that excited interest is the BiCaSrCu.sub.2 O.sub.y, known as 1:1:1:2. The 1:1:1:2 ceramic suffered similarly from the same poor properties exhibited by the initial 1:2:3 find.
Since the Maeda et al. revelations, others have come forward with discoveries of cousins to the 1:1:1:2 material. Komatsu et al. in the Japanese Journal of Applied Physics, 27, 4, April 1988 discloses a glass-ceramic material with a composition from the Maeda family of superconductors. The glass-ceramics introduced by Komatsu are 3:2:2:4 and 3:3:2:4 type Bi-Ca-Sr-Cu-O materials. Komatsu first makes a glass with the material, rapidly quenches the glass, then anneals the glass forming a thin layer of ceramic material on the surface.
Similarly, Akamatsu et al. describe, in the Japanese Journal of Applied Physics, 27 (9) pages L1696--L1698 (September 1988), thick film superconducting compositions in the Bi-Ca-Sr-Cu-O composition system produced by the crystallization of mixed oxide melts on single-crystal MgO substrates. Nevertheless, prior art compositions of these known types tend to exhibit very poor glass stability and are difficult to form as glasses without uncontrolled crystallization.